One torsion damper of the above kind is described in United Kingdom patent specification No. 1 235 826, in which the hub plate is either mounted on the hub for rotation with the latter as in FIG. 1, or else it is mounted on the hub for relative rotation with respect to the hub as in FIG. 7. It has a greater thickness than the guide rings, and its slots are in the form of windows.
In each case, the windows in the hub plate are cut out by a punching operation using a punch and die, which is usually followed by additional treatment such as cementation, nitriding, or induction hardening, so as to give the hub plate the necessary hardness, in particular that required to resist the engagement of the resilient members. When the windows are made in this way, in a press, plastic surface flow, a torn zone and a burr are all formed. In order to improve fatigue resistance, this necessitates the provision of radiused portions at the corners of the windows. This in turn affects the state of the cut surface.
In addition, this punching operation, despite the radiused portions, can give rise to microscopic cracking, which is detrimental to the useful life of the damper. In addition, during the cutting out operation in the press, a force is exerted on the hub plate which can give rise to permanent deformation in the latter. Alternatively or in addition, it may produce some conicity of the cut edges of the window.
In any event, the use of a punching operation to form the windows has its limitations, and cannot be used on very thick hub plates. Finally, for each new cuttingout operation, tooling investment is substantial, and it is not possible to cut out components which have previously been treated so that they have a hardness above a certain value.